Crumbs turns the corner

Crumbs (Crb) helps flies distinguish apical from basal in their epithelia. Now two new studies show that it is required for cell expansion independently of its specification of apical polarity. During cell expansion, the apical domain of photoreceptors elongates perpendicular to the apical–basal axis so that the domain can extend from the top to the bottom of the fly eye. The process may be conserved in humans, consistent with the recent finding that mutations in the human homologue, CRB1, lead to human retinitis pigmentosa.

In the maturing fly eye, the apical domains and rhabdomeres (containing visual pigment) are in the center of each cluster of photoreceptor cells. The rhabdomere is linked by the stalk membrane to the adherens junctions (AJs), which separate the apical and basolateral membranes.

The new studies by...